In order to connect together, substantially coaxially, thermoplastic tubular parts, for example made of polyethylene, a process has already been proposed for welding these tubes, by butting, according to a preset procedure.
It will be recalled that butt-welding of parts does not require any intermediate connecting sleeve (often termed "fitting"), nor a heating resistance arranged in or on the parts to be welded, while in a technique of electrofusion with "fitting", a coiled electrical wire is customarily embedded in the connection (see U.S. Pat. No. 5,130,518 regarding this).
Within the specific framework of butt-welding, and in particular in patents EP-A-0 643 642 or U.S. Pat. No. 5,620,625, it is known in particular:
to arrange the parts with their ends to be welded face to face, PA1 to heat the ends of the parts by way of a heating element, then to allow the heating produced to diffuse internally therein for a total heating time t1 then of internal heating t2, under a pressure P1, P'1 during the heating time and a pressure P2, P'2 during the internal heating time, in order there to obtain beads of material B1, following a substantially coaxial stroke of relative displacement of the parts (C) by way of displacement means, PA1 then, on expiry of a transition time interval t3, to apply the said ends of the parts to one another, by their beads of material, following another relative stroke (C') by way of the said displacement means), for a duration t4 necessary to establish a welding pressure P5, P'5 followed by a time t5 for substantial maintaining of the said welding pressure (normally with deformation then of the said beads; B2), PA1 after which, the weld obtained is maintained substantially without manipulation for a cooling time interval t6. PA1 heating the ends to be welded of the parts by means of a heating element currently termed a "mirror", PA1 supplying electrical energy to the heating element, so as to be able to meet the criteria for heating the said ends of the parts, PA1 additionally providing means for measuring the temperature of the said parts at the start of welding (ambient temperature), the temperature measured being then transmitted to means for implementing the welding programme, in order to adapt the duration of the electrical current applied to the heating element, according to the ambient temperature measured. PA1 pressure(s) P1, P'1, and/or P2, P'2, and/or P5, P'5, PA1 electrical energy to be supplied to the said means for displacement of the parts (voltage, current intensity . . . ), PA1 stroke(s) of relative displacement of the parts, C and/or C', PA1 Time taken to obtain the beads (B1) during the phase of heating the parts in contact with the heating element (t1, t2) and/or t3 (transition time) and/or t4 (time required to reach the welding pressure) and/or t5 (time for which this welding pressure is maintained). PA1 the ambient temperature is measured by means of a temperature probe, PA1 corresponding temperature readings are supplied to a processing unit of a control box which determines the adapted value of the time, electrical energy, pressure and/or stroke in question, according to a correction algorithm previously entered in an internal memory of the box, to a correction coefficient also previously entered in the internal memory of the said box, and to a nominal reference or desired time, pressure, electrical energy and/or stroke(s). PA1 the ambient temperature is still measured by means of a temperature probe or equivalent, PA1 the (each) part to be welded is equipped with an external support designed to contain data concerning the welding of the said parts and capable of being read by reading means suitable for transmitting these data to the processing unit of a control box, PA1 the ambient temperature readings of the probe and the said data read on the (each) support are fed to the said processing unit which determines the adapted value of the time, pressure, electrical energy, (U, I, . . . ) and/or the stroke in question, still according to a correction algorithm, a correction coefficient and a nominal reference or desired time, pressure, electrical energy and/or stroke.
In the two above-mentioned documents, provision is moreover made to adapt the time, by lengthening or shortening it, according to an ambient temperature measured in the environment of the parts, during the course of the welding process.
More precisely, provision is made for: